assassin's shroud

Hi, when fighting eagle shades & legionnaire shades, I cast assassin's shroud on them to prevent them from casting their healing spells. However, it did not work, and they cast their heals multiple times, although they took damage from poison. (Assassin's shroud had worked against me, very nicely, in Marleybone!) I was puzzled, because I don't think I cast it incorrectly. This happened in multiple fights. Anyway, thought you should know.Thank you, Virtuous Anne Radcliffe